Method of manufacturing thermoplastic resin foam particle

ABSTRACT

A method for manufacturing thermoplastic resin expandable granules comprising the steps of: injecting a blowing agent under pressure into a thermoplastic resin which is melted in an extruder; extruding the melted resin containing the blowing agent as an extrudate from a plurality of small holes in a die attached to a distal end of the extruder directly into a coolant liquid, and directly cutting the extrudate by high-speed rotary blades; and cooling and solidifying the extrudate by contacting with the coolant liquid, and thereby obtaining the expandable granules wherein the melted resin containing the blowing agent passing through land parts of the small holes of the die is extruded such that a shearing speed is 12,000 to 35,000 sec −1 , and an apparent melt viscosity of the resin is 100 to 700 poise.

TECHNICAL FIELD

The present invention relates to a method for manufacturingthermoplastic resin expandable granules.

Priority is claimed on Japanese Patent Application No. 2003-324550,filed Sep. 17, 2003, the contents of which are incorporated herein byreference.

BACKGROUND ART OF INVENTION

Generally, a method for impregnating a blowing agent into resin granulesthat are obtained by suspension polymerization is widely adopted as amethod for manufacturing thermoplastic resin expandable granules.However, this method has a problem in that a waste water disposal plantis required for disposing a large quantity of waste water containing asuspension stabilizer, a surface active agent, and a polymerizationinitiator or the like since the polymerization is carried out in a watermedium. Furthermore, the method has a problem in that the yield ofexpandable granules having a predetermined granule size is low becausethe granules obtained by the method vary in size.

As a method for solving these problems, a manufacturing method called anextrusion process, in which a resin melted and mulled in an extruder isextruded from a die that is fitted to the end of the extruder so thatthe resin is cut into granules, is known.

The extrusion process is classified into two types according to thetiming of cutting the resin into granules. One is a method called ahot-cut method, in which the resin is extruded as a string shape fromthe die into pressurized liquid and cut directly by a rotary cutterattached to the die. The other is a method called a cold cut method, inwhich the resin is extruded once in the air as a string shape and thenbrought into coolant liquid, and the string-shaped resin is cut aftercooling while being drawn up from the coolant. In comparison to thelatter (the cold cut method), the former method (the hot-cut method) hasthe advantages in that the productivity of the expandable granules ishigh and the obtained granules are easily handled because sphericalgranules that have no edges can be obtained.

For example, a method for manufacturing the thermoplastic resinexpandable granules by the hot-cut method has been proposed in which athermoplastic resin and a blowing agent that has a boiling point of −50°C. to 0° C. are mulled in an extruder, extruded into water of 20° C. to100° C. and equal to or less than 40 atmospheres, and cut into granulesdirectly in the water (e.g., refer to Patent Document 1).

In addition, a method has been proposed in which a thermoplastic resinand a blowing agent are melted and mulled in an extruder, the resin isextruded into heated and pressurized liquid, and the resin isimmediately cut into granules. Subsequently, the granules are cooledslowly in a pressurized container, the pressure in the container isreleased, and aging the granules under temperature conditions equal toor greater than 40° C. and equal to or less than a temperature 15° C.higher than the low temperature endothermic peak determined by DSCmeasurement (e.g., refer to Patent Document 2).

In addition, as a method for manufacturing thermoplastic resinexpandable granules in which, unlike the processes described above,thermoplastic resin granules that do not contain a blowing agent aremanufactured by a hot-cut method and a blowing agent is subsequentlyimpregnated into the resin granules to manufacture thermoplastic resinexpandable granules, a method has been proposed in which, when thethermoplastic resin is extruded from a nozzle die into water and cut bya cutter blade that rotates while in close contact with a die surface toform a spherical shape, the resin temperature is adjusted such that theviscosity of the melted resin at a die entrance is 100 to 50,000 poiseand the extrusion rate per nozzle hole is 0.1 to 6.0 kg/hr (e.g., referto Patent Document 3).

Alternatively, as a manufacturing method for spherical expandablepolystyrene resin granules in which polystyrene resin granulescontaining a conjugate diene polymer are impregnated with a blowingagent in a water medium, a method has been proposed in which, prior tothe impregnation of the resin granules with the blowing agent, the resinis melted in an extruder, extruded through extrusion holes, and cut. Inthis method, the shearing speed of the resin in the die land part isequal to or greater than 2,500 sec⁻¹ and equal to or less than 10,000sec⁻¹, and the apparent viscosity is equal to or greater than 150 poiseand equal to or less than 700 poise (e.g., refer to Patent Document 4).

Patent Document 1: Japanese Examined Patent Application, SecondPublication No. S48-20423

Patent Document 2: Japanese Unexamined Patent Application, FirstPublication No. H07-314438

Patent Document 3: Japanese Unexamined Patent Application, FirstPublication No. S61-195808

Patent Document 4: Japanese Unexamined Patent Application, FirstPublication No. H09-208735

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

The Patent Documents 1 and 2 teach that when manufacturing theexpandable granules by the hot-cut method, the selection of the blowingagent, the method for cooling the resin immediately after extrusion, andthe aging conditions of the expandable granules after cutting, areimportant. In the methods disclosed in the Patent Documents 1 and 2, theexpansion of the granules cannot be observed during the granulemanufacture, and expandable granules which appear to be spherical andhave a favorable external appearance can be obtained. However, there isa problem in that the mechanical strength of a molded product obtainedby expansion-molding these expandable granules is inferior in comparisonto a molded product that is expansion-molded using expandable granulesobtained by a suspension polymerization impregnation method usingidentical raw materials. This tendency becomes particularly significantwhen the granule diameter is equal to or less than 1.5 mm.

In addition, the Patent Document 3 teaches that when manufacturing theresin granules by using the hot-cut method, the melt viscosity and theresin discharge rate of the resin passing through the die significantlyinfluence the shape of the granules. However, in the Patent Document 3,the range of the disclosed condition that the melt viscosity be 100 to50,000 poise is too broad. When a thermoplastic resin is extruded undergeneral conditions, the melt viscosity naturally falls within thisrange; therefore, particularly when used in foamed products, the meltviscosity range in which expandable granules that have a superiorstrength can be obtained, is not defined.

In addition, the Patent Document 4 teaches that when manufacturing resingranules by using a hot-cut method, the shearing speed and the meltviscosity of the resin passing through the die significantly influencethe shape of the granules. However, even if the shearing speed isadjusted so as to fall in a range between 2,500 sec⁻¹ and 10,000 sec⁻¹,an expandable granule having superior strength when used in foamedproducts, could not be obtained.

Specifically, the melt viscosity of the resin becomes significantlylower when a blowing agent is present therein, and thus the suggestionsfor resin viscosity and shearing speed disclosed in the Patent Documents3 and 4, which are technologies in which the resin is extruded withoutimpregnating a blowing agent, cannot serve as references when extrudingexpandable granules.

In consideration of the problems described above, an object of thepresent invention is to provide a manufacturing method for thermoplasticresin expandable granules that have a spherical shape and uniformdiameter, and enables the manufacturing of foamed products having asuperior mechanical strength.

Means for Solving the Problem

In order to attain the objects described above, the present invention isa manufacturing method for thermoplastic resin expandable granules inwhich: a blowing agent is injected under pressure into a thermoplasticresin that has been melted in an extruder; the melted resin containingthe blowing agent is extruded directly into a coolant liquid through alarge number of small holes of a die that is installed on a distal endof the extruder, this extrudate is cut by high-speed rotary blades; andthe extrudate is cooled and solidified by bringing the extrudate intocontact with a liquid to obtain expandable granules, wherein the resinis extruded so that the shearing speed of the melted resin containingthe blowing agent is 12,000 to 35,000 sec⁻¹, and the apparent meltviscosity of the resin is 100 to 700 poise when the resin passes throughland parts of the small holes of the die.

Preferably, in this method, the diameter of the small hole is 0.5 to 1.0mm, and the die is used whose small holes have a land length of 2 to 4mm.

Preferably, in this method, when a polystyrene resin is used as thethermoplastic resin, the resin temperature at an access into the die isset in a range between 150° C. and 180° C.

Preferably, in this method, when a polyethylene resin is used as thethermoplastic resin, the resin temperature at an access into the die isset in a range between 130° C. and 160° C.

Preferably, in this method, when a polypropylene resin is used as thethermoplastic resin, the resin temperature at an access into the die isset in a range between 180° C. and 210° C.

EFFECTS OF THE INVENTION

According to the method of the present invention, it is possible tocarry out continuously melting of the resin, adding the blowing agent,mulling, cooling, and granulation by using an extruding and hot-cutmethod, and it is possible to manufacture expandable granules having auniform granule diameter efficiently.

In addition, in the method of the present invention, when extruding themelted resin containing the blowing agent, by controlling the shearingspeed and the melt viscosity of the resin in the land parts of smallholes of the die within a particular range, it is possible tomanufacture stably expandable granules from which an foamed product canbe obtained that has a mechanical strength equivalent to an foamedproduct obtained by expandable granules that are obtained by thesuspension polymerization impregnation method, which is difficult in theconventional hot-cut method.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an example of a manufacturingapparatus that is used to implement a manufacturing method forthermoplastic resin expandable granules of the present invention.

FIG. 2 is a longitudinal cross-sectional view showing an example of adie used in the manufacturing apparatus.

FIG. 3 is a graph showing a relationship between the molecular weight ofthe resin used for manufacturing the expandable granules and thestrength of a molded product.

FIG. 4 is a graph showing a relationship between the resin temperatureduring the manufacture of the expandable granules and the strength of amolded product.

FIG. 5 is a graph showing a relationship between the resin meltviscosity during the manufacture of the expandable granules and thestrength of a molded product.

FIG. 6 is a graph showing a relationship between the shearing speedduring the manufacture of the expandable granules and the strength of amolded product.

FIG. 7 is a cross-sectional view showing another example of a die usedin the manufacturing apparatus.

FIG. 8 is a view taking along a line II-II of FIG. 7.

FIG. 9 is an exploded view showing a tubular conduit portion of the dieshown in FIG. 7.

BRIEF DESCRIPTION OF THE REFERENCE

-   1: extruder-   2, B: die-   3, 112: cutting chamber-   4: water-feeding pump-   5: dewatering-and-drying device-   6: water tank-   7: chamber-   11: raw material supply hopper-   12: blowing agent supply opening-   13: high pressure pump-   31: cutter-   101: die holder-   102: melted resin conduit-   103: die holder part heater-   104: bolt-   105: die main body-   105 a: resin-discharging surface-   106: rod heater-   107: tubular conduit-   108: small hole-   109: cutter rotating shaft-   110: cutter blade support member-   111: cutter knife-   113: circulating liquid inlet-   114: circulating liquid outlet-   115 a: heating medium inlet-   116 a: heating medium outlet-   117: heating medium conduit-   P, Q, and R: area

BEST MODE FOR CARRYING OUT THE INVENTION

Below, the present invention will be explained with reference to thedrawings.

The present invention was completed based on the knowledge thatsignificant differences occur when expandable granules, which have beenobtained by systematically varying the shearing speed and the apparentmelt viscosity of the resin when the resin passes through the die, areexpansion-molded into various plate shapes and their strength ismeasured.

In the method of the present invention, a blowing agent is injectedunder pressure into a thermoplastic resin that has been melted in anextruder, the melted resin containing the blowing agent is extrudeddirectly into a coolant liquid from a large number of small holes in adie installed on a distal end of the extruder, an extrudate is cut as itis extruded by high-speed rotary blades, and the extrudate is cooled andhardened by contacting with a liquid to obtain expandable granules. Theextrudate is extruded so that, when passing through land parts of thesmall holes of the die, the shearing speed of the resin is 12,000 to35,000 sec-1 and the apparent melt viscosity is 100 to 700 poise.

A feature of the present invention is in the point that when thethermoplastic resin containing the blowing agent is extruded from thesmall holes, the shearing speed and the apparent melt viscosity of theresin in the land parts of the small holes are strictly controlled, andthis control becomes possible only by simultaneously adjusting both thestructure of the die, in particular, the diameter and the land length ofthe small holes, and adjusting the resin temperature during extrusion.

In the die suitable for use in the present invention, the diameter ofthe small holes from which the resin is discharged is 0.5 to 1.0 mm andthe land length “a” (refer to FIG. 2) of the small holes is 2 to 4 mm.

FIG. 1 shows an example of the manufacturing apparatus used to implementthe manufacturing method for the thermoplastic resin expandable granulesof the present invention. The manufacturing apparatus is constituted of:an extruder 1 that has a raw material supply hopper 11 and a blowingagent supply opening 12 that is connected to a high pressure pump 13; adie 2 that is installed on a distal end of the extruder 1; a cuttingchamber 3 that rotatably accommodates a cutter 31 that is in closecontact with a resin-discharging surface of the die 2 and has an inletand an outlet for a circulating liquid; a water-feeding pump 4 thatfeeds the circulating liquid to the cutting chamber 3; adewatering-and-drying device 5 to which the expandable granules and thecirculating liquid are fed and that carries out solid-liquid separation;a water tank 6 that stores the circulating liquid; and a chamber 7 intowhich the dried expandable granules are conveyed.

In the method of the present invention, a general-purpose extruder canbe used as the extruder 1. For example, a single shaft extruder, atwo-shaft extruder, or two single shaft extruders connected together maybe used, or a two-shaft extruder in a first stage connected to asingle-shaft extruder in a second stage may be used.

FIG. 2 is a drawing showing a longitudinal sectional view of the die 2that is used in this manufacturing apparatus. In FIG. 2, the referencenumeral 21 denotes a resin pressure detecting part, 22 denotes smallholes, 23 denotes a die face surface, and “a” denotes the land length ofthe small holes. The thermoplastic resin heated and melted in theextruder 1 is mulled with a blowing agent that is injected underpressure from the blowing agent supply opening 12, pumped to the die 2after being cooled, and extruded from the small holes 22 into thecutting chamber 3. The melted resin containing the blowing agent isextruded from the small holes 22 to contact the circulating liquidinside the cutting chamber 3, cut into a short flake shape by the cutter31, formed into a spherical shape in the liquid, and cooled. Theexpandable granules that have been formed into a spherical shape in thecirculating liquid are sent to the dewatering-and-drying device 5, andafter the expandable granules have been separated from the circulatingliquid, they are dried and stored in the chamber 7.

In the die 2, the shearing speed in a die land part through which themelted resin containing the blowing agent passes is calculated by thefollowing equation 1:τ=4q/(π·r3)  (1)

Here, “τ” denotes the shearing speed (sec⁻¹), “q” denotes the volume ofthe resin discharge rate (cm³/sec) per hole, “π” denotes the ratio ofthe circumference of a circle to the diameter, and “r” denotes theradius (cm) of the small holes.

In addition, the apparent melt viscosity of the resin is calculated bythe following equation 2:η=(ΔP·π·g·r4·ρ)/(8Q·L)  (2)

Here, “η” denotes the apparent melt viscosity (kg/(cm·sec)), ΔP denotesthe pressure loss (kg/cm2) of a small holes land part, “π” denotes theratio of the circumference of a circle to the diameter, “g” denotes thegravitational acceleration (cm/sec2), “r” denotes the radius (cm) of thesmall holes, “ρ” denotes the resin density (kg/cm3), Q denotes the resinmass discharge rate per small hole (kg/sec), and L denotes the landlength (cm) of the small holes.

More concretely, the detected pressure at the position shown byreference numeral 21 in FIG. 2 is used as the value of ΔP, and the valueof “g” is 9,800 cm/sec2. In addition, because the small holes 22 of thedie 2 do not necessarily all discharge the resin effectively, theeffective number of holes for calculating “q” and Q is calculated bymeasuring the mass of 1,000 obtained expandable granules, and theaverage value thereof serves as the actual granule mass. The number ofholes effectively working is calculated by comparing the actual granulemass with a theoretical granule mass value. The theoretical granule massvalue is calculated from the mass of the resin supplied to the extruder1 per hour and the number of cutting per hour by the cutter 31 (therotation number per hour×the number of cutters), assuming that all ofthe small holes 22 are effectively working.

According to the equation (1), the shearing speed “r” is proportional tothe resin quantity “q” that is discharged from one small hole 22, andinversely proportional to the third power of the radius “r” of the smallholes 22. Here, the shearing speed of the resin when passing through thesmall holes 22 influences the shape of the obtained granules. In orderto obtain granules having a true spherical shape with uniform diameters,it is necessary to maintain the shearing speed in a range of 12,000 to35,000 sec-1. When the shearing speed is less than 12,000 sec-1, thegranules acquire a distorted shape, and when these granules areexpanded, the expanded granules acquire a flattened disc shape. Inaddition, when the shearing speed exceeds 35,000 sec-1, cutting by thecutter 31 is not favorable, burr-shaped projections are produced on thegranules, and powdering of the resin also occurs frequently.

Here, the rate of discharge of the resin from the small holes 22 can besimply adjusted by using a die 2 whose number of holes depends on theamount of resin supplied per hour to the extruder 1, and the diameter ofthe small holes 22 may be 0.5 to 1.0 mm. When the diameter exceeds 1.0mm, it becomes difficult to adjust the shearing speed equal to orgreater than 12,000 sec-1, and alternatively, when less than 0.5 mm, itbecomes difficult to adjust the shearing speed equal to or less than35,000 sec-1. This is not preferable.

In addition, in order to obtain spherical expandable granules from whicha foamed product having a superior mechanical strength is obtained byexpansion, it is necessary to maintain the apparent melt viscosity ofthe resin when passing through the small holes 22 between 100 to 700poise. According to the above equation (2), the apparent melt viscosityis proportional to the pressure loss ΔP while the resin is passingthrough the small holes 22, that is, the resin pressure in the die.

The adjustment of this resin pressure is carried out by setting thetemperature of the resin during extrusion. The higher the temperaturebecomes, the lower the resin pressure becomes, which in turn causes themelt viscosity to become low. In addition, the resin melt viscosityduring extrusion can also be adjusted by the land length “a” of thesmall holes 22. In order to simplify the adjustment of the meltviscosity, preferably the land length “a” of the small hole 22 is 2 to 4mm. When the land length “a” exceeds 4 mm, the resin pressure at thesmall holes 22 becomes high, and thereby the resin temperatureadjustment for maintaining the proper melt viscosity range becomesdifficult, which is not preferable. In addition, when the land length“a” is less than 2 mm, the flow of the resin in the hole portiondeteriorates, and the shape and the size of the cut granules becomeuneven, which is not preferable.

When the melt viscosity is less than 100 poise, suppressing theexpansion of the granules during cutting becomes difficult, adhesionbetween the cut granules is caused, and when these granules areexpansion-molded, only a molded product having a weak mechanicalstrength can be obtained. Alternatively, when the melt viscosity exceeds700 poise, although granules become ones from which the foamed productsobtained by expansion-molding have a superior mechanical strength whenexpanded, the shape becomes distorted, and the expanded granules thatexpand into these foamed products become flat granules. Thus, the fillerproperties deteriorate when filled into molds during molding, the sizeand the shape of the expanded granules that appear at the surface of themolded products are uneven, and therefore the external appearance is notpreferable. The range of the apparent melt viscosity is more preferably200 to 500 poise.

Examples of thermoplastic resins that can be used in the presentinvention, while not particularly limited, are: polystyrene,styrene/butadiene copolymers, styrene/methacrylate copolymers,styrene/maleic anhydride copolymers, aromatic vinyl resins such as ASresins and ABS resins, vinyl chloride resins such as polyvinyl chloride,polyvinylidene chloride, vinyl chloride/vinyl acetate copolymers; olefinresins such as polyethylene, polypropylene, polybutene,polyethylene-vinyl acetate copolymers; acrylic resins such as polymethylacrylate, polyethyl acrylate, and methyl methacrylate/styrenecopolymers; polyester resins such as polyethylene terephthalate andpolybuthylene-terephthalate; amide resins such as polycaprolactone andpoly hexamethylene adipamide; and separately or in compounds ofpolyurethane, polycarbonate, polyetherimide, polyphenylene ether, andpoly lactic acid. Among these, aromatic vinyl resins and olefin resinsare particularly advangateous.

Examples of blowing agents that can be used in the present inventionare: aliphatic hydrocarbons such as propane, normal butane, isobutene,normal pentane, isopentane, neopentane, and cyclopentane; ethers such asdimethyl ether and diethyl ether; various alcohols such as methanol andethanol; and carbonic acid gas, nitrogen, and water. Among these, thealiphatic hydrocarbons are advantageous, and furthermore, separately orin compounds, normal butane, isobutene, normal pentane, or isopentaneare particularly advantageous. The added amount of the blowing agent canbe increased or decreased depending on the target expansion ratio of theexpandable granules, but generally, a range of 2 to 15 parts by weightper 100 parts by weight of resin is preferable.

Preferably, a bubble nucleating agent, which is for adjusting thebubbles when the expandable granules expand, is compounded into theresin composition used in the present invention. Examples of the bubblenucleating agent that can be used are powders such as talc, calciumcarbonate, magnesium carbonate, diatom earth, calcium stearate,magnesium stearate, barium stearate, zinc stearate, aluminum stearate,silica, and Teflon (trademark) powder, in addition to sodiumbicarbonate, citric acid, and azodicarboxamide. However, among these,preferably 0.2 to 2.0 parts by weight powdered talc is added to theresin. In addition, a flame retardant, an antistatic agent, a coloringagent or the like may be added.

The temperature of the melted resin differs depending on the type ofresin that is used, and preferably the temperature of the expandableresin in the access into the die is adjusted to 50 to 100° C. above themelting point of the resin. In addition, it is necessary to extrude theresin directly into a liquid so that the extruded string-shapedexpandable resin does not expand. Preferably, the temperature of theliquid used for this resin coolant is set 100 to 200° C. lower than theresin temperature at the access into the die.

For example, if the resin to be used is a styrene resin, it is necessaryto heat the resin to about 200 to 230° C. so that it will meltcompletely in the extruder 1. However, the temperature of the resin inthe access into the die is preferably as low as possible within therange in which the flow of the resin does not deteriorate, preferablyabout 150 to 180° C. The temperature of the liquid used to cool theresin at this time is preferably 30 to 60° C. Warm water is suitable asthe liquid.

At this time, when the temperature of the liquid is too high, thegranules expand during cutting and adhere together, which is notpreferable. Alternatively, when the temperature of the liquid isextremely low, the resin in the small holes 23 solidifies and theclogging of the small holes 22 occurs, which is not preferable.

In the case in which the resin to be used is a polyethylene resin (theproportion of the polyethylene in the resin as a whole is equal to orgreater than 50%), the temperature of the resin in the access into thedie is preferably about 130 to 160° C., and the temperature of theliquid used to cool the resin at this time is preferably 20 to 50° C.

Furthermore, in the case in which the resin to be used is apolypropylene resin (the proportion of the polypropylene to the resin asa whole is equal to or greater than 50%), the temperature of the resinin the access into the die is preferably about 180 to 210° C., and thetemperature of the liquid used to cool the resin at this time ispreferably 40 to 70° C.

The expandable resin that has been extruded directly into the resincoolant liquid is cut immediately after being extruded into the liquidby the cutter that rotates in close contact with the die face surfaceand cooled to form expandable granules. In this manner, based oncontrolling the extrusion conditions within an appropriate range, themanufactured granules are almost completely spherical and have diametersslightly larger than the diameter of the holes in the die. Theexpandable granules are conveyed along with water through conduits, andafter being dewatered and dried, are made into a product.

To manufacture foamed products by molding the thermoplastic resinexpandable granules obtained by the method described above in a mold, itis possible to use conventional well-known expansion-molding methods andapparatuses. For example, in the case in which the resin is a styreneresin, the pre-expanded granules, which have been obtained by expandingthe expandable granules 10 to 100 times by using steam, are aged acertain amount of time and subsequently are filled into a mold andreheated using steam to obtain the foamed products having the desiredshape.

According to the method of the present invention, it is possible tocarry out continuously, using the extrusion hot-cut method the meltingof the resin, adding the blowing agent, mulling, cooling, andgranulation; and it is possible to manufacture efficiently expandablegranules having uniform grain diameters.

In addition, in the method of the present invention, when extruding themelted resin containing the blowing agent, by controlling the shearingspeed and the melt viscosity of the resin in the land parts of smallholes of the die within a predetermined range, it is possible tomanufacture stably expandable granules from which foamed products havinga mechanical strength equivalent to foamed products obtained byexpandable granules that are obtained by using a suspensionpolymerization method are obtained, and which was difficult when usingthe conventional hot-cut method.

TEST EXAMPLE

Below, the effect of the present invention is clarified by test examplescomparing test examples of the present invention with comparativeexamples made using the conventional method.

In the following test examples and comparative examples, the values ofthe sphericity of the pre-expanded granules and the density and theflexural strength of the foamed products are measured respectively bythe following methods.

<Sphericity of the Pre-Expanded Granules>

To find the sphericity of the pre-expanded granules, the pre-expandedgranules obtained by expanding the expandable granules were positionedin a vernier caliper, the grain diameter was measured from variousangles, the largest diameter W1 and the smallest diameter W2 wereextracted, and the sphericity was calculated by using the followingequation (3):K=W1/W2  (3)

In addition, the sphericity was evaluated in the following manner:

Evaluation Value of K

∘ equal to or greater than 1.0 and less than 1.3

Δ equal to or greater than 1.3 and less than 1.6

x equal to 1.6 or greater

<Molded Product Density>

The density of the molded product was measured using the methoddisclosed in “Expanded Plastics and Rubbers: Measuring the ApparentDensity” in JIS K7222: 1999. Specifically, a 10×10×5 cm test piece froma molded product was cut such that the cell structure of the test piecewas not altered, the mass thereof was measured, and the density obtainedusing the following equation (4):Density(g/cm3)=test piece mass(g)/test piece volume(cm3)  (4)

<Flexural Strength of the Molded Product>

The flexural strength was measured by the method disclosed in “ExpandedPlastics Heat Insulating Materials”, JISA9511: 1999. Specifically, usinga Tensilon general testing machine UCT-10T (made by Orientec), the testpiece size was 75×300×15 mm, the compression rate was 10 mm/min, theassembly had a 10R loading nose and 10R supports, and the distancebetween the supports was 200 mm. The measurement was carried out, andthe flexural strength was calculated by using the following equation:Flexural strength(MPa)=3FL/2bh2  (4)

(Here, F denotes the largest flexural load (N), L denotes the distance(mm) between supports, “b” denotes the width (mm) of the test piece, and“h” denotes the thickness (mm) of the test piece.)

Test Example 1

In this test example, the expandable granules according to the presentinvention were manufactured by using the apparatus shown in FIG. 1.

In a tumbler, 0.3 parts by weight of powdered talc was mixed in advancewith 100 parts by weight of polystyrene resin (Toyo Styrene; HRM10N).This mixture was continuously supplied to a single-shaft extruder havinga 90 mm opening diameter at a rate of 100 kg per hour. The temperaturein the extruder was set to the highest temperature of 210° C. After theresin was melted, 6 parts by weight of isopentane as the blowing agentwas injected under pressure into the resin in the course of theextrusion. The resin and blowing agent were mulled and cooled in theextruder. While the temperature of the resin at the end of the extruderwas maintained at 170° C. and the pressure at the resin access into thedie was maintained at 14 MPa, the blowing agent-containing melted resinwas extruded via a die into the cutting chamber communicating with thedie and in which 40° C. water was circulating, and simultaneously, theextrudate was cut by a high speed rotating cutter that has 10 bladesarranged in the circumferential direction. The die has 150 small holeshaving a diameter of 0.6 mm and a land length “a” of 3.5 mm. The cutgranules were cooled, dewatered, and dried to obtain the expandablegranules. The obtained expandable granules had no deformations, burrs,or the like, and the granules had almost perfect spherical shapes withdiameters of approximately 0.8 mm. At this time, when the shearing speedand the apparent melt viscosity of the melted resin containing theblowing agent when passing through the land parts of small holes of thedie is calculated based on the equations described above, the shearingspeed was 15,608 sec⁻¹, and the apparent melt viscosity was 360 poise.

After aging for 72 hours, the obtained expandable granules were heatedfor two minutes in a box-shaped blower at 0.5 kg/cm² and becamepre-expanded granules. After resting for 24 hours, the pre-expandedgranules were filled into a 300×400×50 mm mold. The foamed product wasmolded by blowing steam into the mold for 20 seconds at a gauge pressureof 1.0 kg/cm². At this time, the sphericity of the pre-expanded granuleswas 1.15, the density of the obtained foamed products was 0.025 g/cm³,and the flexural strength was measured at 0.65 MPa.

Test Example 2

In a test example 2, the same resins and composition were adopted as thetest example 1. The equipment of the test example 2 is the same as thatof the test example 1 excepting the modified die. The expandablegranules were manufactured while the extrusion conditions were adjusted,and these expandable granules were pre-expanded and molded in a moldsimilarly to the test example 1. The sphericity of the obtainedpre-expanded granules and the density and flexural strength of themolded products were measured. The specifications for the die, theextruding conditions, the sphericity of the pre-expanded granules, andthe density and flexural strength of the molded products are shown inTABLE 1.

Comparative Examples 1 to 3

In comparative examples 1 to 3, the same resins and composition wereadopted as the test example 1. The equipment of the test example 2 isthe same as that of the test example 1, excepting a modified die. Theexpandable granules were manufactured while the extrusion conditionswere adjusted, and these expandable granules were pre-expanded andmolded in a mold similarly to the test example 1. The sphericity of theobtained pre-expanded granules and the density and flexural strength ofthe molded products were measured. The specifications for the die, theextruding conditions, the sphericity of the pre-expanded granules, andthe density and flexural strength of the molded products are shown inTABLE 1 and TABLE 2.

Test Example 3

In a test example 3, the same apparatus as the test example 1 was used,but the resin was altered to the mixture of polyethylene resin (NihonPolyolefin KK: JE 111D) and polystyrene resin (Toyo Styrene KK: HRM10N),mixed respectively at 60/40 mass ratio, and the expandable granules wereobtained. The obtained expandable granules were made into pre-expandedgranules in a heating-and-steaming container immediately aftermanufacture, and subsequently, a foamed product was obtained by the samemethod as that in the test example 1. The sphericity of the pre-expandedgranules and the density and flexural strength of the molded productswas measured. The specifications for the die, the extrusion conditions,the sphericity of the pre-expanded granules, and the density andflexural strength of the molded products are shown in TABLE 1.

Test Example 4

In a test example 4, the same apparatus as the test example 1 was used,but the resin was altered to the mixture of polypropylene resin(Mitsui-Sumitomo Polyolefin KK: S131) and polystyrene resin (ToyoStyrene KK: HRM10N) mixed respectively at 60/40 mass ratio, and theexpandable granules were obtained. The obtained expandable granules weremade into pre-expanded granules in a heating-and-steaming containerimmediately after manufacture, and subsequently, a foamed product wasobtained by the same method as that in the test example 1. Thesphericity of the pre-expanded granules and the density and flexuralstrength of the molded products was measured. The specifications for thedie, the extrusion conditions, the sphericity of the pre-expandedgranules, and the density and flexural strength of the molded productsare shown in TABLE 1.

Comparative Example 4

In a comparative example 4, a composition of resin is same as that inthe test example 1, but the resin was extruded without injecting theblowing agent under pressure and cut into resin granules. Next, theresin granules were placed in a pressurized container, isobutene,serving as the blowing agent, was added at 15 parts per weight to 100parts per weight of resin granules in a water medium having a dispersingagent and a plasticizer (toluene), and a blowing agent impregnationtreatment was carried out for 4 hours at 70° C. The resin granules werecooled to room temperature and dewatered; then the expandable granuleswere obtained. These expandable granules were made into foamed productsby the same method as that in the test example 1. The density and theflexural strength of the foamed product are shown in TABLE 2.

Test Example 5

In this test example 5, the same apparatus and the equipment as the testexample 1 were used excepting the die was modified into a die B whichhas a countermeasure for preventing the clogging of small holes 108through which the resin was discharged, and the expandable granules weremanufactured while adjusting the extrusion conditions.

The die B used in this test example is shown in FIG. 7 to FIG. 9. In thedie B shown in FIG. 8, 12 tubular conduits (resin conduits) 107 in theareas Q and one small hole 108 for each of the tubular conduits 107 areshown. However, in the die B used in this test example, 16 tubularconduits 107 are provided in the areas Q, and 10 small holes 108 areprovided for each of the tubular conduits 107.

This die B has a die holder 101 that is fixed to a distal end of theextruder (not illustrated) and a die main body 105 that is fixed to adistal end of the die holder 101. An interior of the die holder 101which forms a tube, serves as a melted resin conduit 102 thatcommunicates with the end of the extruder. Reference numeral 103 denotesa die holder part heater, and reference numeral 104 denotes a bolt forattaching the die main body 105. A plurality of rod heaters 106 isinserted into the die main body 105. In this die B, resin passes fromthe end of the extruder through the melted resin conduit 102 and thetubular conduits 107, and is extruded from the plurality of small holes108 provided on each resin-discharging surface 105 a.

A cutter that has a cutter rotating shaft 109, a cutter blade supportmember 110, and cutter knives 111, is accommodated in the cuttingchamber 112 that is connected to a resin-discharging surface 105 a ofthis die B, and the cutting chamber 112 has a circulating liquid(coolant liquid) inlet 113 and a circulating liquid outlet 114. In thiscutting chamber 112, the cutters are rotated in the circulating liquidwater, the resin discharged from the resin-discharging surface 105 a isimmediately cut in the water, and the obtained granules are conveyed outfrom the circulating liquid outlet 114 along with the flowing water(circulating liquid).

The die main body 105 has the plurality of the tubular conduits 107 thatcommunicate with the melted resin conduit 102 and that communicate withthe plurality of small holes 108 that open into the resin-dischargingsurfaces 105 a of the die main body 105 therein. The tubular conduits107 are provided on a circle defined on the resin-discharging surfaces105 a. However, as shown in FIG. 8, the small holes 108 and the tubularconduits 107 are not provided in areas P (preferably having a centralangle of 10° to 50°) in the inflow direction and the outflow directionof the water stream (the coolant liquid stream) of the resin-dischargingsurface 105 a, and the areas R (preferably, having a central angle of10° to 50°), which is orthogonal thereto. The small holes 108 are formedonly in areas Q, and are not formed in areas P and R. As can beunderstood from FIG. 8, a resin-discharging surface 105 a of the diemain body 105 is circular, and the small holes 108 are arranged on anideal circle that has a diameter that is smaller than the outercircumference of the resin-discharging surface 105 a and is concentricto the resin-discharging surface 105 a only in the areas Q, excludingthe vertical and horizontal areas P and R, when viewed from the centerof the circle.

The bottom of the resin-discharging surface 105 a aligns with thedirection of the circulating liquid inlet 113, the top of theresin-discharging surface 105 a aligns with the direction of thecirculating liquid outlet 114, and the left-and-right directions of theresin-discharging surface 105 a align with the direction orthogonal tothe direction that connects the circulating liquid inlet 113 and thecirculating liquid outlet 114.

In the die B, the small holes 108 that discharge the resin have adiameter of 0.6 mm and a land length “a” of 3.5 mm, and a total of 160small holes 108 are arranged on a circle defined on theresin-discharging surface 105 a. The small holes 108 are not provided ineither the areas P (having a central angle of 25°), which lie in theinflow direction 113 and the outflow direction 114 of the water(circulating liquid) that fills the cutting chamber 112 in contact withthe resin-discharging surface 105 a, or the areas R (having a centralangle of 25°), which lie in the direction orthogonal to the inflowdirection 113 and the outflow direction 114 of the water. A heatingmedium conduit 117 for heating the resin in the small holes 108 isprovided in the die B. A heating medium inlet 115 a that communicateswith this heating medium conduit 117 is provided on the upper and lowerareas P on the resin-discharging surface 105 a. A heating medium outlet116 a that communicates with the heating medium conduit 117 is providedon the right and left areas R on the resin-discharging surface 105 a.

In the present test example, the resin was supplied to the extruder at arate of 120 kg per hour, the highest temperature of the extruder was setto 220° C., 6 parts per weight of isopentane based on the weight of theresin as a blowing agent was added and mixed into the resin, and theresin was introduced into the die B while maintaining the resintemperature at the distal end of the extruder at 168° C. While thermaloil (heating medium) at 230° C. was circulated in the heating mediumconduit 117 of the die, the resin was extruded into the cutting chamber112 in which 40° C. water circulated and simultaneously cut byhigh-speed rotary cutters identical to those in the test example 1 toobtain the expandable granules.

The resin pressure in the access into the die was 12 MPa. The obtainedexpandable granules were almost true spherical granules having adiameter of 0.7 mm, and the point was ascertained that there were fewercloggings of the small holes than the test example 1 in terms of therelationship between the number of cuts with respect to theresin-discharging rate and the granule diameter, and the resin pressurein the access into the die. When the shearing speed and the apparentmelt viscosity of the melted resin when passing through the die werecalculated based on the above equations, the shearing speed was 13,691sec-1 and the apparent melt viscosity was 352 poise.

Pre-expanding and molding of the obtained expandable granules wascarried out using the method identical to that of the test example 1,and the sphericity of the obtained pre-expanded granules and the densityand flexural strength of the molded products were measured. Themeasurements are shown along with the extrusion conditions in TABLE 3.TABLE 1 Test Comparative Comparative example 1 Test example 2 example 1example 2 Used raw material (% wt) PS (100%) PS (100%) PS (100%) PS(100%) Mold Hole diameter (mm) 0.6 0.5 0.5 0.8 Hole numbers (unit) 150200 200 100 Land length (mm) 3.5 3.5 3.5 5.0 Blowing agent TypeIsopentane Isopentane Isopentane Isopentane Impregnated amount (% wt) 66 5 5 Resin temperature (° C.) 170 172 210 170 Pressure at access into14 21 21 21 metal mold (MPa) Cutting chamber water 40 40 40 40temperature (° C.) shearing speed (sec⁻¹) 15,608 31,007 36,445 13,158apparent melt viscosity 360 240 210 894 (poise) Pre-expanded granule1.15 ◯ 1.10 ◯ 1.38 Δ 1.82 X sphericity Molded product strength 0.0250.024 0.030 0.026 (g/cm³) Molded product flexural 0.65 0.58 0.42 0.70strength (MPa)

TABLE 2 Comparative Comparative Example 3 Test example 3 Test example 4example 4 Used raw material (% wt) PS (100%) PE (60%) PP (60%) PS (100%)PS (40%) PS (40%) Mold Hole diameter (mm) 0.4 0.6 0.6 — Hole numbers(unit) 240 150 150 — Land length (mm) 3.0 3.5 3.5 — Blowing agent TypeIsopentane Isopentane Isopentane Isobutane Impregnated amount (% wt) 6 66 15 (after impregnation) Resin temperature (° C.) 214 152 202 —Pressure at access into metal 24 12 18 — mold (MPa) Cutting chamberwater 40 30 60 — temperature (° C.) shearing speed (sec⁻¹) 45,304 18,72132,445 — apparent melt viscosity 160 240 370 — (poise) Pre-expandedgranule (burrs) X 1.18 ◯ 1.25 ◯ — sphericity Molded product strength0.025 0.036 0.040 0.025 (g/cm³) Molded product flexural 0.38 0.52 0.460.56 strength (MPa)

TABLE 3 Test example 5 Used raw material (% wt) PS (100%) Metal moldhole diameter (mm) 0.6 hole number (units) 160 land length (mm) 3.5blowing agent type Isopentane impregnation amount (parts by weight) 6Resin temperature (° C.) 168 Pressure at access into mold (MPa) 12Cutting chamber water temperature (° C.) 40 shearing speed (sec⁻¹)13,691 apparent melt viscosity (poise) 352 Pre-expanded granulessphericity 1.10 ◯ Molded product density(g/cm³) 0.025 Molded productflexural strength (MPa) 0.070

According to the results shown in TABLE 1 and TABLE 2, the pre-expandedgranules made from the expandable granules manufactured according to themethods and apparatus of the test examples 1 and 2 according to thepresent invention exhibit an advantageous sphericity. In addition, whenthe obtained granules are made into foamed products, the molded productflexural strength is strong, and has a strength that is not inferior tothe molded products made of the expandable granules obtained by themethod shown in the comparative example 4, in which the granules areimpregnated after formation. In addition, the expandable granulesmanufactured in the test examples 3 and 4, although they cannot becompared directly because the resin compositions of the expandablegranules were different from that of the comparative example 4, have agranule shape and molded product strength of practical use.

In contrast, in the comparative example 1 and the comparative example 3,because the shearing speed of the resin at the land parts was too high,the produced expandable granules tended to have some foam andburr-shaped projections, and thus the strength of the foamed productswas also extremely weak.

In comparative example 2, because the apparent melt viscosity of theresin in the die land part was too high, the produced expandablegranules were flat, the sphericity of the pre-expanded granules wasextremely poor, and when these were molded, the size of the expandedgranules at the surface of the molded product was uneven, and thus theexternal appearance deteriorated.

As shown in TABLE 3, in the test example 5, it is possible tomanufacture expandable granules stably over a long period of time, andat the same time, the obtained expandable granules had a highsphericity, and when these expandable granules were expansion-molded,molded products having a superior strength were obtained. That is, thedie B used in the test example 5 was effective in preventing clogging ofthe small holes 108 through which the resin is discharged, and as aresult, it was understood that it is possible to maintain a low diepressure during extrusion. Note that in this die B, it was possible toobtain the effect of clogging prevention by providing small holes 108 atleast one but not both of the positions in the inflow direction 113 andthe outflow direction 114 of the water stream and the position in thedirection orthogonal to the inflow direction 113 and the outflowdirection 114 of the water stream.

The influence of Each of the Parameters on the Foamed Product Strength

FIG. 3 to FIG. 6 are graphs showing the result of investigating theinfluence that each of the parameters had on the strength (maximumflexural load) of the molded products obtained by expansion-molding theexpandable granules when the expandable granules are manufactured byusing a polystyrene resin identical to that in the test example 1. Theparameters were the molecular weight of the resin, the resin meltviscosity, and the shearing speed.

FIG. 3 is a graph showing the relationship between the molecular weightof the resin used in the manufacture of the expandable granules and thestrength of the molded products. Concerning the range of the molecularweights shown in FIG. 3, a clear interrelationship between the molecularweight of the resin and the molded product strength could not be found.

FIG. 4 is a graph showing the relationship between the temperature ofthe resin during the manufacture of the expandable granules and thestrength of the molded product. As shown in FIG. 4, a tendency could beobserved in which the strength of the molded product increased as thetemperature of the resin decreased.

FIG. 5 is a graph showing the relationship between the resin meltviscosity (apparent melt viscosity) during the manufacture of theexpandable granules and the strength of the molded product. As shown inFIG. 5, a tendency could be observed in which the strength of the moldedproduct increased as the resin melt viscosity increased.

FIG. 6 is a graph showing the relationship between the shearing speedduring the manufacture of the expandable granules and the strength ofthe molded product. As shown in FIG. 6, a tendency could be observed inwhich the strength of the molded product increased as the shearing speeddecreased.

From the results shown in FIG. 3 to FIG. 6, it can be understood thatthe resin melt viscosity (apparent melt viscosity) and the shearingspeed during manufacture of the expandable granules influence thestrength of the molded product obtained by expansion-molding theseexpandable granules. In the present invention, when a melted resincontaining a blowing agent that has been mulled in an extruder isextruded from the small holes of a die, by extruding the melted resinsuch that the shearing speed is 12,000 to 35,000 sec-1 and the apparentmelt viscosity of the resin is 100 and 700 poise, the unique effectswere obtained that the sizes of the obtained expandable granules wereuniform and the expandable granules were easily handled. In addition,the further unique effect was obtained that the strength of the moldedproduct obtained by expansion-molding was not inferior to that of theexpandable granules obtained by the suspension polymerizationimpregnation method. As shown in FIG. 5 and FIG. 6, the ranges of theapparent melt viscosity and the shearing speed in the present inventionare set based on the results of thorough investigations in order tobalance well the advantageous characteristics such as the sphericity ofthe obtained expandable granules, and not simply set such that thestrength of the molded product becomes high. Thereby, these numericalranges have a sufficiently critical significance.

INDUSTRIAL APPLICABILITY

Because the method for manufacturing thermoplastic resin expandablegranules of the present invention continuously carries out a processfrom the resin supply to obtaining the expandable granules and uses amanufacturing apparatus having a simple structure, the industrial valueis extremely high. In addition, because the granule size of the obtainedexpandable granules is uniform, the expandable granules are easilyhandled, and the strength of the molded products obtained byexpansion-molding is not inferior to that of expandable granulesobtained by suspension polymerization impregnation method, the methodcan be used extensively in molding, for example, shock-resistantwrapping material for household electrical appliances and precisionmachinery, transportation containers for food and machine parts, ashock-absorbing parts for vehicle parts, structural parts, displayparts, leisure parts and the like. The method thereby has a highindustrial value.

1. A method for manufacturing thermoplastic resin expandable granulescomprising the steps of: injecting a blowing agent under pressure into athermoplastic resin which is melted in an extruder; extruding the meltedresin containing the blowing agent as an extrudate from a plurality ofsmall holes in a die attached to a distal end of the extruder directlyinto a coolant liquid, and directly cutting the extrudate by high-speedrotary blades; and cooling and solidifying the extrudate by contactingwith the coolant liquid, and thereby obtaining the expandable granuleswherein the melted resin containing the blowing agent passing throughland parts of the small holes of the die is extruded such that ashearing speed is 12,000 to 35,000 sec⁻¹, and an apparent melt viscosityof the resin is 100 to 700 poise.
 2. A method for manufacturingthermoplastic resin expandable granules according to claim 1, whereinthe small holes of the die have a diameter of 0.5 to 1.0 mm, and a landlength of the small holes is 2 to 4 mm.
 3. A method for manufacturingthermoplastic resin expandable granules according claim 1 or 2, whereinthe thermoplastic resin is a polystyrene resin, and a temperaturethereof at an access into the die is set to a range of 150 to 180° C. 4.A method for manufacturing thermoplastic resin expandable granulesaccording to claim 1 or 2, wherein the thermoplastic resin is apolyethylene resin, and a temperature thereof at an access into the dieis set to a range of 130 to 160° C.
 5. A method for manufacturingthermoplastic resin expandable granules according to claim 1 or 2,wherein the thermoplastic resin is a polypropylene resin, and atemperature thereof at an access into the die is set to a range of 180to 210° C.
 6. A method for manufacturing thermoplastic resin expandablegranules according to claim 1 or 2, wherein: a die is used in which: aresin-discharging surface contacting with a stream of the coolant liquidis provided; and at the resin-discharging surface, there are no smallholes in at least one of a position in an inflow direction and outflowdirection of the stream of the coolant liquid and a position orthogonalto the inflow direction and the outflow direction of the stream of thecoolant liquid; and the method comprises the steps of: supplying thethermoplastic resin to the extruder to which the die being attached, andmelting and mulling the thermoplastic resin in the extruder; moving thethermoplastic resin towards the die while injecting a blowing agent intothe thermoplastic resin to form a resin containing a blowing agent; andcutting the thermoplastic resin containing the blowing agent dischargedfrom the small holes of the die with a cutter into the stream of thecoolant liquid.
 7. A method for manufacturing thermoplastic resinexpandable granules according to claim 6, wherein the die comprises: aresin conduit communicating with a cylinder of the extruder and thesmall holes formed in the die; and a heating medium conduit heating theresin in the resin conduit, and wherein the resin in the resin conduitis heated by the heating medium flowing through the heating mediumconduit.
 8. A method for manufacturing thermoplastic resin expandablegranules according to claim 7, wherein an inlet and an outlet of theheating medium conduit are provided in the resin-discharging surface inproximity to a position at which the small holes are not provided.